PHP Magic Methods in JavaScript

readme.md

JavaScript Magic Methods

This script implements some of PHP's magic methods for JavaScript classes, using a Proxy.

The fork contains a modification that makes the __get method usable recursively like in php

Example

You can use it like this:

const Foo = magicMethods(class Foo {
  constructor () {
    this.bar = 'Bar'
  }
  
  __get (name) {
    switch (name){
      case 'firstName': return 'Petra';
      case 'lastName': return 'Mustermann';
      // __get also fires within __get
      case 'fullName': return this.firstName + ' ' + this.lastName;
    }
    console.error('undefined prop ' + name);
  }
})

const foo = new Foo
foo.fullName; // Petra Mustermann

If you're using a JavaScript transpiler like Babel with decorators enabled, you can also use the magicMethods function as a decorator:

@magicMethods
class Foo {
  // ...
}

Supported Magic Methods

Given a class Class and an instance of it, the following are the magic methods supported by this script:

__get(name)

Called when trying to access instance[name] where name is not an existing property of instance.

Attention: As in PHP, the check if name exists in instance does not use any custom __isset() methods.

__set(name, value)

Called when trying to do instance[name] = ... where name is neither set as a property of instance.

__isset(name)

Called when trying to check existance of name by calling name in instance.

__unset(name)

Called when trying to unset property name by calling delete instance[name].

Additional Methods

The following magic methods are made available by this script, but are not supported in PHP:

static __getStatic(name)

Like __get(), but in the Class instead of the instance.

static __setStatic(name, value)

Like __set(), but in the Class instead of the instance.

Why is Magic Method X not supported?

They are either not necessary or not practical:

• __construct() is not needed, there's JavaScript's constructor already.
• __destruct(): There is no mechanism in JavaScript to hook into object destruction.
• __call(): Functions are first-class objects in JavaScript. That means that (as opposed to PHP) an object's methods are just regular properties in JavaScript and must first be obtained via __get() to be invoked subsequently. So to implement __call() in JavaScript, you'd simply have to implement __get() and return a function from there.
• __callStatic(): As in __call(), but with __getStatic().
• __sleep(), __wakeup(): There's no builtin serialization/unserialization in JavaScript. You could use JSON.stringify()/JSON.parse(), but there's no mechanism to automatically trigger any methods with that.
• __toString() is already present in JavaScript's toString()
• __invoke(): JavaScript will throw an error if you'll try to invoke a non-function object, no way to avoid that.
• __set_state(): There's nothing like var_export() in JavaScript.
• __clone(): There's no builtin cloning functionality in JavaScript that can be hooked into.
• __debugInfo(): There's no way to hook into console.log() output.

Can I extend a class with Magic Methods on it?

Yes:

// `Bar` inherits magic methods from `Foo`
class Bar extends Foo {}

Or, if class Bar contains magic methods itself:

const Bar = magicMethods(class Bar extends Foo {
  // You may define `Bar`'s magic methods here
})

magic-methods.js

function magicMethods (clazz) {
  // A toggle switch for the __isset method
  // Needed to control "prop in instance" inside of getters
  let issetEnabled = true

  const classHandler = Object.create(null)

  // Trap for class instantiation
  classHandler.construct = (target, args, receiver) => {
    // Wrapped class instance
    const instance = Reflect.construct(target, args, receiver)

    // Instance traps
    const instanceHandler = Object.create(null)
    
    // __get()
    // Catches "instance.property"
    const get = Object.getOwnPropertyDescriptor(clazz.prototype, '__get')
    if (get) {
      instanceHandler.get = (target, name, receiver) => {
        // We need to turn off the __isset() trap for the moment to establish compatibility with PHP behaviour
        // PHP's __get() method doesn't care about its own __isset() method, so neither should we
        issetEnabled = false
        const exists = Reflect.has(target, name)
        issetEnabled = true
        
        if (exists) {
          return Reflect.get(target, name, receiver)
        } else {
          return get.value.call(instanceProxy, name)
        }
      }
    }

    // __set()
    // Catches "instance.property = ..."
    const set = Object.getOwnPropertyDescriptor(clazz.prototype, '__set')
    if (set) {
      instanceHandler.set = (target, name, value, receiver) => {
        if (name in target) {
          Reflect.set(target, name, value, receiver)
        } else {
          return target.__set.call(target, name, value)
        }
      }
    }

    // __isset()
    // Catches "'property' in instance"
    const isset = Object.getOwnPropertyDescriptor(clazz.prototype, '__isset')
    if (isset) {
      instanceHandler.has = (target, name) => {
        if (!issetEnabled) return Reflect.has(target, name)
        
        return isset.value.call(target, name)
      }
    }

    // __unset()
    // Catches "delete instance.property"
    const unset = Object.getOwnPropertyDescriptor(clazz.prototype, '__unset')
    if (unset) {
      instanceHandler.deleteProperty = (target, name) => {
        return unset.value.call(target, name)
      }
    }
    // keep instanceProxy to be used in get trap when __get method is called
    const instanceProxy = new Proxy(instance, instanceHandler)
    return instanceProxy
  }

  // __getStatic()
  // Catches "class.property"
  if (Object.getOwnPropertyDescriptor(clazz, '__getStatic')) {
    classHandler.get = (target, name, receiver) => {
      if (name in target) {
        return target[name]
      } else {
        return target.__getStatic.call(receiver, name)
      }
    }
  }

  // __setStatic()
  // Catches "class.property = ..."
  if (Object.getOwnPropertyDescriptor(clazz, '__setStatic')) {
    classHandler.set = (target, name, value, receiver) => {
      if (name in target) {
        return target[name]
      } else {
        return target.__setStatic.call(receiver, name, value)
      }
    }
  }

  return new Proxy(clazz, classHandler)
}